The present invention relates to a passive infrared intrusion detection system and, more particularly, to such a system for ceiling mounting. In its more specific aspects, the invention is concerned with such a system wherein a single detector may be used to view downwardly through an expanded 360.degree. field.
Passive infrared intrusion detection systems are well known in the art. Typically, an infrared intrusion system comprises a fresnel lens having a plurality of segments, each segment for focusing infrared radiation from a zone in a volume of space onto an infrared detector. U.S. Pat. No. 4,757,204 shows such a system wherein a plurality of such fresnel lenses are arranged in an inverted dome-like configuration to focus infrared radiation from a zone extending 360.degree. around the detector.
In another type of prior art passive infrared intrusion detection system, a mirror having a plurality of segments receives infrared radiation from a plurality of spaced apart zones and reflects radiation and focuses it from the zones onto a single detector. U.S. Pat. No. 3,703,718 shows such a system. wherein FIG. 6 illustrates an arrangmeetn desigend for perimeter viewing and ceiling mounting.
The prior art also teaches passive infrared intrusion systems wherein a combination of fresnel lenses and mirrors is used to focus and to reflect infrared radiation from different zones onto a detector. Such a system wherein the fresnel lens focuses infrared radiation in zones which are far away from the detector and the mirror reflects and focuses infrared radiation from zones which are near to the detector is shown in U.S. Pat. No. 4,841,284.
The present invention provides a ceiling mounted passive infrared intrusion detection system which achieves an expanded 360.degree. viewing zone through means of rings of mirrors which are mounted beneath the detector and disposed to detect infrared radiation from multiple segments within a volume of space beneath the detector and focus the reflected radiation onto the detector. The system is compact and thus ideally suited for incorporation into a single housing which may embody a secondary detection system, such as a microwave system. In the preferred embodiment, the system of the invention employs multiple concentric rings of mirrors, each of which is comprised of multiple facets. Each facet views a segment of space and the rings are so arranged as to provide a pattern of viewing which intercepts substantially the entire area to be monitored by the system. The segments of space viewed by the respective rings of mirrors are radially spaced. In one embodiment designed to avoid the possibility that detection might be foiled by a fast moving body, adjacent facets of the mirrors within the rings are paired to view overlapping segments of space.
The rings of mirrors in the present system are radially spaced to provide a path through which focused reflected radiation may pass to the detector. The innermost ring of mirrors has an open center through which the detector of the system may view directly, thus providing viewing coverage immediately below the system, without the interposition of reflecting means. In the preferred embodiment, the rings of mirrors are also so spaced as to provide a shorter focal length for mirror facets disposed to view space closest to the detector.
A principal object of the invention is to provide a new and improved passive infrared detection system suitable for ceiling mounting which has an expanded coverage pattern that is not a function of the field of view of the pyroelectric detector used in the system.
Another object of the invention is to provide such a system which is compact and relies upon reflection to increase the field of view of the detector.
Still another object of the invention is to provide such a system wherein the expanded field of view of the detector is effectively covered with a pattern of viewing segments and wherein the segments produce an elongate pattern upon intersecting a body.
A further object of the invention is to provide such a system wherein a single dual element pyroelectric detector may be used to monitor an expanded volume of space disposed in an area 360.degree. around and below the detector.
Still a further object of the invention is to provide such a detector wherein infrared radiation is gathered and focused by rings of mirrors comprised of multiple facets and certain of the facets are paired to provide overlapping viewing segments.
Another and more specific object of the invention is to provide such a system wherein the viewing segments closest to the detector spread out more quickly than those further from the detector.
Another specific object related to the latter object is to provide such a system wherein such spreading is provided by providing a shorter focal length for the viewing segments closer to the detector and the center of the protected area.
These and other objects will become more apparent when viewed in light of the following detailed description and accompanying drawings.